A variable-gain amplifier (VGA) is an amplifier that varies its gain depending on a control voltage. A transconductance amplifier (a.k.a. gm amplifier) puts out a current proportional to its input voltage. The transconductance of a MOSFET transistor is the change in its drain current divided by the small change in the gate/source voltage with a constant drain/source voltage.
There are a variety of known techniques to improve the linearity of amplifiers. For example, one technique has been to combine PFET and NFET amplifier topologies thereby superposing their transconductances on one another. This technique tends to be bandwidth-limited because of the characteristics of the PFET.
Another technique, known as derivative superposition, combines saturation-region-biased FETs and triode-region-biased FETs. This technique requires different device sizes and gate bias voltages for the saturation-region-biased FETs and the triode-region-biased FETs. In addition, the area and current for the triode-region-biased FETs do not contribute to gain since the primary purpose of triode-region-biased FETs is to improve linearity.
Yet another technique is to introduce negative feedback using inductor degeneration to linearize the transconductance of amplifier.
Referring to FIG. 1, a schematic is illustrated of a prior art technique to improve the linearity in a low noise amplifier (LNA). A differential feedback-transformer has electro-magnetic coupling between its primary inductors 102a and 102b and its degeneration inductors 104a and 104b, respectively, to provide negative feedback.
FIG. 2 illustrates a simplified layout of the differential feedback-transformer depicted in FIG. 1. Notably, an additional balun transformer is required to acquire a single-ended output signal from the differential output Vout in circuit 100 of FIG. 1, thus requiring additional integrated circuit area.